Hydraulic elevators have been used for many years. Before electric elevators were practical, hydraulic elevators were commonly operated by water, utilizing the pressure in city water mains and accumulators moving full load down at about 500 FPM. When reliable electric motors became available, the water driven hydraulic elevators were replaced over the years with oil driven systems in which hydraulic fluid (oil) was pumped from a reservoir, a supply tank, to the cylinder of the elevator to raise the ram, hence the elevator car. The fluid was then exhausted through a valve which could be opened and closed at a predetermined rate, but which was opened to a fixed effective area whenever it was opened. Under these circumstances the rate of descent of the car depended upon the head of oil and the weight of the car. In any event it has been slow with an empty car and relatively fast with full load. Conventionally, a hydraulic elevator descends at the rate of about 100 feet per minute with empty car.
The sytem of the present application is related in function to the system of patent Ser. No. 3,056,469, which makes use of a hydraulic system automatically to adjust the applied voltage on a generator shunt field so that the generator will generate the exact voltage to move the elevator at a constant rate of speed regardless of load on the elevator, up to 2000 feet per minute. The physical movement of the elevator and speed of the elevator are hydraulically computed together in order to generate a synchro signal.
One of the objects of this invention is to provide a hydraulic elevator system in which the maximum rate of descent is uniform, regardless of the head of fluid or the weight on the elevator car.
Another object is to provide such a system which is simple, economical, safe, effective, and versatile.
Still another object is to provide such a system that can easily be retrofitted at little expense and effort.
Other objects will become apparent to those skilled in the art in the light following and accompanying drawings.